feat(billing): period usage aggregation by metric function

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

fusion_centralize_billing/models/usage.py

@@ -58,3 +58,27 @@ class FusionBillingUsage(models.Model):
                 existing.write({'quantity': quantity})
                 return existing
         return self.create(vals)
+
+    @api.model
+    def _aggregate(self, subscription, metric, period_start, period_end):
+        """Aggregate stored usage for a subscription+metric within [period_start, period_end)
+        using the metric's aggregation function."""
+        rows = self.search([
+            ('subscription_id', '=', subscription.id),
+            ('metric_id', '=', metric.id),
+            ('period_start', '>=', period_start),
+            ('period_end', '<=', period_end),
+        ])
+        qtys = rows.mapped('quantity')
+        if not qtys:
+            return 0.0
+        agg = metric.aggregation
+        if agg == 'sum':
+            return sum(qtys)
+        if agg == 'max':
+            return max(qtys)
+        if agg == 'last':
+            return rows.sorted('period_start')[-1].quantity
+        if agg == 'unique_count':
+            return float(len(set(qtys)))
+        return sum(qtys)

fusion_centralize_billing/tests/test_usage.py

@@ -31,3 +31,22 @@ class TestUsageIngestion(TransactionCase):
         rows = self.Usage.search([('idempotency_key', '=', k)])
         self.assertEqual(len(rows), 1)        # no duplicate
         self.assertEqual(rows.quantity, 175.0)  # last value wins for the same key
+
+    def test_aggregate_sum(self):
+        for i, q in enumerate([10.0, 20.0, 30.0]):
+            self.Usage._record_usage(self.sub, 'cpu_seconds', q,
+                                     '2026-05-01', '2026-06-01', idem='cpu-%d' % i)
+        total = self.Usage._aggregate(self.sub, self.metric, '2026-05-01', '2026-06-01')
+        self.assertEqual(total, 60.0)
+
+    def test_aggregate_max(self):
+        self.metric.aggregation = 'max'
+        for i, q in enumerate([10.0, 55.0, 30.0]):
+            self.Usage._record_usage(self.sub, 'cpu_seconds', q,
+                                     '2026-05-01', '2026-06-01', idem='m-%d' % i)
+        self.assertEqual(self.Usage._aggregate(self.sub, self.metric, '2026-05-01', '2026-06-01'), 55.0)
+
+    def test_aggregate_excludes_other_periods(self):
+        self.Usage._record_usage(self.sub, 'cpu_seconds', 99.0, '2026-04-01', '2026-05-01', idem='apr')
+        self.Usage._record_usage(self.sub, 'cpu_seconds', 5.0, '2026-05-01', '2026-06-01', idem='may')
+        self.assertEqual(self.Usage._aggregate(self.sub, self.metric, '2026-05-01', '2026-06-01'), 5.0)